Ancient Hominid Study Reveals Early Similarities Between Humans And Apes

April 12, 2013

Image Caption: (Left Section) This image shows a reconstruction of Au. Sediba, with a small-bodied female modern H. sapiens on left and a male chimpanzee on right. (Right Section) This image shows a larger reconstruction of Au. sediba. Credit: Lee Berger; University of the Witwatersrand

One of the largest studies on some of the most complete remains of early human ancestors has culminated in a comprehensive look into how an early hominid (Australopithecus sediba) moved and chewed. The study, collaborated on by an international team of scientists and published in six papers in the journal Science, details not only early traits but also describes notable features that set it apart from modern humans.

The research has focused on the remains of two individuals taken from the site of Malapa, about 30 miles northwest of Johannesburg, South Africa. The site, discovered in August 2008, has yielded more than 200 bones of at least five early hominids. The fossils at the Malapa site date 1.977 to 1.98 million years ago, according to researchers.

The 2008 discovery was led by Prof. Lee Berger of University of Witwatersrand´s (Wits) Evolutionary Studies Institute (ESI) in South Africa. Berger is also coauthor on the latest study, and has described the research as an “unprecedented insight into the anatomy and phylogenetic position of an early human ancestor.” The sediba species was also named by Berger and his colleagues in 2010.

The six papers appearing in the April 12, 2013 issue of Science are:

“Dental morphology and the phylogenetic “place” of Australopithecus sediba,” led by Joel D. Irish of Liverpool John Moores University (LJMU), UK.

“Mandibular remains support taxonomic validity of Australopithecus sediba,” led by Darryl J. de Ruiter of Texas A&M University (TAMU).

“Mosaic morphology in the thorax of Australopithecus sediba,” led by Peter Schmid of University of Zurich, Switzerland.

“The vertebral column of Australopithecus sediba,” led by Scott A. Williams of New York University (NYU).

“The lower limb and the mechanics of walking in Australopithecus sediba,” led by Jeremy M. De Silva of Boston University (BU), MA.

An introduction to the six papers is entitled “The Mosaic Nature of Australopithecus sediba,” authored by Prof. Berger.

DENTAL MORPHOLOGY

In the dental morphology study, the authors found evidence that Au. sediba is also a close relative of the previously identified Au. africanus. The new research shows that both species share a closer relation to humans than other australopiths from east Africa.

“Our study provides further evidence that sediba is indeed a very close relative of early humans, but we can’t definitively determine its position relative to africanus,” said Debbie Guatelli-Steinberg, co-author of the study and professor of anthropology at The Ohio State University (OSU).

The study team extended their work by comparing sediba teeth to eight other African hominin species, including modern humans and extinct species of the genus Homo, Australopithecus and Paranthropus. In all, more than 340 fossils and 4,571 recent specimens were examined, including teeth from 44 gorillas as a comparison.

The team focused on 22 separate traits of tooth crowns and roots that can give clues of the relationship between different species. The team used standardized measurements from the Arizona State University (ASU) Dental Anthropology System to compare the teeth for all 22 traits.

They found that on 15 of the traits measured, sediba and africanus scored the same. Also, sediba shared 13 traits with the early human species Homo erectus. As well, sediba and africanus shared five dental traits that were not found in earlier australopiths, further indicating a close relationship. Five traits were also found to be shared with the early humans H. habilis and rudolfensis which were not shared with earlier ancestors, further demonstrating the close connection between sediba, africanus and the first humans.

Gautelli-Steinberg noted that teeth studies provide a good way to examine relationships between different species. One reason is that they are well-preserved in the fossil record, allowing for comparisons of large samples for many ancient specimens.

The study team also noted that most of the dental traits used in the analysis did not have a selective advantage that could have helped one species survive over another, meaning that finding similar traits between two species makes a confident statement that such traits had not evolved independently.

The new dental traits study supports the other research on sediba, said Gautelli-Steinberg. “All of the research so far shows that sediba had a mosaic of primitive traits and newer traits that suggest it was a bridge between earlier australopiths and the first humans,” she added.

Gautelli-Steinberg explained that the dental analysis shows that “both africanus and sediba are more closely related to humans than the famous “Lucy” skeleton fossil found in East Africa in 1974.” The fossil, which represents Au. afarensis, was once thought to be the closest relative of modern humans.

“Our research on teeth can’t definitively settle if either sediba or africanus is more closely related to humans than the other species,” Guatelli-Steinberg said. “But our findings do suggest that both are closely related to each other and are more closely related to humans than afarensis.”

“We need to find more sediba remains to help fill in the missing pieces of this evolutionary puzzle.”

VERTEBRAL COLUMN MORPHOLOGY

NYU´s Scott Williams, an anthropologist at the University´s Center for the Study of Human Origins and lead author on the vertebral column study of Au. sediba, said: “The abundance and remarkable preservation of fossils from Malapa provide unique insights into the way this fascinating extinct species interacted with and moved around in its environment.”

Williams and colleagues analyzed elements of the cervical, thoracic, lumbar, and sacral regions of the vertebral columns in Au. sediba. The study revealed the species had a human-like curvature of the lower spine, but was functionally longer and more flexible than the spine of the modern human.

“They probably walked in a way that we might find strange–a ‘compromise’ form of bipedalism indicative of a hominin that still partially relied on climbing trees,” Williams explained in a statement.

The evidence is based on an “adult female” with the most “intact terminal thoracic region” to date in an early hominin remain. The evidence “provides critical information on the transition in inter-vertebral joints, and, by inference, mobility of the lower back,” Williams further explained.

“The bony spine of Au. sediba likely had the same number of vertebrae as that of modern humans,” Williams added. “However, it differed in one primary aspect of its configuration–the transition in inter-vertebral articular facets occurred at a higher level of the spine that than normally occurs in modern humans. In combination with other features, a functionally longer lower back would have allowed for a more flexible spine in Au. sediba relative to that of modern humans.”

Morphological indicators in the strong curvature of the lumbar spine suggest that Au. sediba and africanus evolved similarly and more closely to the recent Nariokotome H. erectus — the most complete early human skeleton known, to date.

THORAX MORPHOLOGY

Peter Schmid of the University of Zurich led a study to learn more about the morphology of the thorax of Au. sediba.

The thorax of the narrow upper ribcage in sediba is more similar to large apes, such as orangutans and gorillas, than it is to the human thorax, which is uniformly cylindrical. He explained that a conical ribcage with a raised shoulder joint gives sediba a look that resembles a permanent shrug, like what might be observed in large apes. But the lower thorax indicates a slim waist, like that of a modern human.

The narrow upper thorax of apes allows for movement of the shoulder blade, an important trait for climbing and swinging from branch to branch in trees, said Schmid. However, the conical shape of the thorax makes it difficult for sediba to swing its arms when walking upright or when running.

And since the conical thorax was similar in length to that of large apes, Schmid assumes that sediba was not able to walk or run on both feet as well as humans. “They probably couldn’t run over longer distances, especially as they were unable to swing their arms, which saves energy,” he added.

MORE RESEARCH

Among the other papers in the large international study, one shows how the lower extremities of the heel, metatarsus, knee, hips and back of sediba are unique to the species. The study described how the specimen must have walked, with feet turned sharply inward. Lead researcher Jeremy De Silva of Boston University and colleagues explained that this inward turn distinguishes it from other australopiths, drawing a conclusion that our ancestors moved around in a much different way.

In the paper on the upper limb morphology of Au. sediba, Steven Churchill of Duke University and colleagues noted that sediba must have been an experienced climber. Evidence of this comes from extensive research of the upper arm, radius, ulna, scapula clavicle and sternum that was discovered in Malapa in the 2008 expedition.

The remains were clearly pinpointed to a single individual, which the researchers said were unique in the entire previously known fossil record of the earliest hominins. With the exception of the hand bone, the upper extremity is very original. Its arms were adapted for climbing as well as brachiation (arm swinging).

Prof. Berger from ESI said that the studies have provided a “glimpse of a hominin species that appears to be mosaic in its anatomy and that presents a suite of functional complexes that are both different from that predicted for other australopiths, as well as that for early Homo.”

“Such clear insight into the anatomy of an early hominin species will clearly have implications for interpreting the evolutionary processes that affected the mode and tempo of hominin evolution and the interpretation of the anatomy of less well preserved species,” he added.

“Discoveries such as Australopithecus sediba and the Malapa site demonstrate the need for further African based exploration in the rich fossil fields of southern Africa, and additionally demonstrate the tremendous promise of the palaeosciences on the continent,” concluded Berger.